Expandable thermoplastic resin beads

ABSTRACT

A process for producing expandable thermoplastic resin beads which comprises suspending in an aqueous medium interpolymer beads comprising a polyphenylene ether resin and a polymerized vinyl aromatic monomer such as styrene, polymerizing the vinyl aromatic monomer in the presence of a polymerization catalyst to interpolymerize the vinyl aromatic monomer with the polyphenylene ether and, optionally, adding a cross-linking agent, to form interpolymerized thermoplastic resin beads, and introducing a blowing agent under pressure into the thermoplastic resin beads. The resulting resin beads have excellent foamability and molding fusability, and a foamed shaped article having superior thermal stability can be prepared from these beads.

This is a divisional of application Ser. No. 502,095, filed on Mar. 30,1990 now U.S. Pat. No. 4,968,466, which is a divisional of applicationSer. No. 384,774, filed July 24, 1989 now U.S. Pat. No. 4,920,153, whichis a divisional of Ser. No. 232,900, filed on Aug. 16, 1988 now U.S.Pat. No. 4,874,796, which in turn is a division of Ser. No. 062,004,filed on June 12, 1987 now U.S. Pat. No. 4,782,098.

This invention relates to a process for producing expandablethermoplastic resin beads, especially those which have superiorfoamability and molding fusability and the resulting foamed articlesmade from such beads provide good thermal stability.

BACKGROUND OF THE INVENTION

Generally, it is easy to obtain polystyrene beads having a highexpansion ratio. The resulting foamed articles made from such beads havehigh rigidity and good shape retention, but have the disadvantage inthat they are fragile and have poor chemical resistance, oil resistanceand thermal stability. In pre-expanded bead form, polystyrene has aglass transition temperature of, for example, 80°-95° C., precluding itsuse in automotive foams, for example, under the hood.

Foamed products of polystyrene and styrene-maleic anhydride are known,e.g., from U.S. Pat. No. 4,442,232 and, although they have higherthermal resistance, they are rather difficult to prepare, and havelimitations in their impace resistance and compressive strength.Expandable thermoplastic resin beads comprising ethylene-propylenecopolymers grafted with vinyl aromatic monomers are also known, e.g.,from Kajimura et al., U.S. Pat. No. 4,303,756. The compositions whichare produced are said to have excellent thermal stability, butresistance to solvents and oxidation tend to be lower than desirable. Toovercome these drawbacks, it has also been proposed to form foams fromblends of polyphenylene ethers and polystyrene or high impact, i.e.,rubber-modified grafted polystyrene imbibed with liquid blowing agents.Mention can be made of U.S. Pat. No. 3,492,249, which suggests foaming aphysical blend of polyphenylene ether and polystyrene. However, todevelop maximum strength, the cells have to be elongated and involves ahot-stretching step, which is not desirable. In U.S. Pat. Nos. 4,598,100and 4,598,101, blends of a polyphenylene ether resin and high impactpolystyrene are imbibed at atmospheric pressure with a volatilechlorinated hydrocarbon in an extruder, and the blend is extrudedthereafter into a foam; U.S. Pat. Nos. 4,532,263 and 4,598,104, discloseimpregnating pellets of a blend of polyphenylene ether and high impactpolystyrene, preexpanding the pellets, and then shaping them in an openmold to form foam. The last-mentioned patent also discloses that foamedsheets of blended polyphenylene ether resin an polystyrene can bethermoformed into shaped foamed articles. In all cases, the prior artcompositions of the polyphenylene ether resin do not provide ultimateresistance to thermal and physical shocks because they either containrubber modified polystyrene and they always are merely physical blendsof the polymer components. In all cases, the expanded pellets are not ofoptimum size for molding.

A new method of making expandable particles and foams from polyphenyleneethers has now been discovered. This involves interpolymerization of avinyl aromatic monomer with a preformed polyphenylene ether resin withthe object to make a low density higher temperature resistant expandablebead foam, the beads having a smaller size and more spherical shape thanpreviously obtainable. When, for example, foamable particles are madefrom a polymerized vinyl monomer, e.g., styrene, containingpolyphenylene ether resin, e.g., poly(2,6-dimethyl-1,4-phenylene) ether,the particulate product comprises an interpolymer and thus is differentin this respect from the pelletized blends employed in the prior art.The interpolymer has the significant and unexpected ability to increasethe glass transition temperature of the preexpanded beads to 104°-117°C., typically, and the high temperature stability of the ultimate foamscan be increased to or higher than a previously attainable degree merelyby increasing the polyphenylene ether content of the small, spericalparticles of interpolymer used to make the foam. Other physicalproperties are improved, as well, especially, uniform cell structure andfoam strength.

SUMMARY OF THE INVENTION

According to the invention, there is provided a process for producingexpandable thermoplastic resin beads which comprises suspending in anaqueous medium from 1 to 50 parts, preferably 5 to 40 parts, by weightof a polyphenylene ether resin and 99 to 50 parts, preferably 95 to 60parts, by weight of a vinyl aromatic monomer per 100 parts by weight ofresin and monomer; adding a polymerization catalyst and polymerizing thevinyl monomer to form interpolymerized thermoplastic resin beads andduring interpolymerization or subsequently impregnating theinterpolymerized thermoplastic resin beads preferably under pressurewith an easily volatilizable hydrocarbon or halogenated hydrocarbonblowing agent. The blowing agent can be imbibed during or afterformation of the interpolymer, for example, in a pressure vessel or inan extruder. The particles can also be melted, for example, in anextruder and then imbibed.

This invention also provides compositions comprising discrete particlesof such interpolymers of a polyphenylene ether resin and a polymerizedvinyl aromatic monomer imbibed under pressure with an easilyvolatilizable hydrocarbon or halogenated hydrocarbon blowing agent, theblowing agent being present in an amount sufficient to foam theinterpolymer to a density less than about 20 lbs./ft³.

In another aspect the invention contemplates a process for forming ashaped polymer foam structure comprising:

(a) imbibing particles of such an interpolymer of a polyphenylene etherresin and a polymerized vinyl aromatic monomer under pressure with aneasily volatilizable hydrocarbon or halogenated hydrocarbon blowingagent;

(b) heating the imbibed particles to a temperature sufficient to causeexpansion of said particles to a density significantly less than that ofthe imbibed particles; and

(c) filling a mold with the expanded particles and subjecting theparticles to sufficient heat to fuse the particles together on coolingto form a shaped coherent foam structure.

A further feature of the invention are such interpolymers in foam formhaving a density of less than about 20 lbs/ft³, wherein saidinterpolymer is a blowing agent imbibable composition comprising apolyphenylene ether resin and a polymerized vinyl aromatic monomer,wherein said blowing agent is an easily volatilizable hydrocarbon orhalogenated hydrocarbon.

DETAILED DESCRIPTION OF THE INVENTION

In the process of this invention, the polyphenylene ether resin is usedas a nucleus into which the vinyl aromatic monomer is absorbed and thevinyl aromatic monomer is interpolymerized with the resin in thepresence of a polymerization catalyst. The polyphenylene ether resins(also known as polyphenylene oxides) used in the invention are a wellknown class of polymers which have become very useful commercially as aresult of the discovery by Allan S. Hay of an efficient and economicalmethod of production See, for example, U.S. Pat. Nos. 3,306,874 and3,306,875. Numerous modifications and variations have since beendeveloped but, in general, they are characterized as a class by thepresence of arylenoxy structural units. The phenolic end groups can alsobe capped, by known procedures, such as reaction with ester-forming andether-forming reagents. The present invention includes all suchvariations and modifications, including but not limited to thosedescribed hereinafter.

The polyphenylene ethers favored for use in the practice of thisinvention generally contain structural units of the following formula##STR1## in which each of these units independently each Q¹ is hydrogen,halogen, primary or secondary lower alkyl (i.e., alkyl containing up to7 carbon atoms), phenyl, haloalkyl or aminoalkyl wherein at least twocarbon atoms separate the halogen or nitrogen atom from the benzenering, hydrocarbonoxy, or halohydrocarbonoxy wherein at least two carbonatoms separate the halogen and oxygen atoms; and each Q² isindependently hydrogen, halogen, primary or secondary lower alkyl,phenyl, haloalkyl, hydrocarbonoxy or halohydrocarbonoxy as defined forQ¹. Examples of suitable primary lower alkyl groups are methyl, ethyl,n-propyl, n-butyl, isobutyl, n-amyl, isoamyl, 2-methylbutyl, n-hexyl,2,3-dimethylbutyl, 2-, 3- or 4-methylphenyl and the corresponding heptylgroups. Examples of secondary lower alkyl groups are isopropyl,sec-butyl and 3-pentyl. Preferably, any alkyl radicals are straightchain than branched. Most often, each Q¹ is alkyl or phenyl, especiallyC₁₋₄ alkyl, and each Q² is hydrogen.

The preferred polyphenylene ethers comprise units of the formula##STR2## wherein the oxygen ether atom of one unit is connected to thebenzene nucleus of the next adjoining unit, n is a positive integer andis at least 50, and each Q is a monovalent substituent selected from thegroup consisting of hydrogen, halogen, hydrocarbon radicals free of atertiary alpha-carbon atom, halohydrocarbon radicals having at least twocarbon atoms between the atom and the phenyl nucleus, hydroxcarbonoxyradicals, and halohydrocarbonoxy radicals having at least two carbonatoms between the halogen atom and the phenyl nucleus.

Both homopolymers and copolymers are included. Suitable homopolymers arethose containing, for example, 2,6-dimethyl-1,4-phenylene ether units.Suitable copolymers include random copolymers containing such units incombination with, for example, 2,3,6-trimethyl-1,4-phenylene etherunits. Many suitable random copolymers, as well as homopolymers, aredisclosed in the patent literature, including various Hay patents. Alsocontemplated are graft copolymers, including those prepared by grafingonto the polyphenylene ether chain such vinyl monomers as acrylonitrileand vinyl aromatic compounds, for example, styrene, and such polymers aspolystyrene and elastomers. Still other suitable polyphenylene ethersare the coupled polyphenylene ethers in which the coupling agent isreacted with the hydroxy groups of the two polyphenylene ether chains toincrease the molecular weight of the polymer. Illustrative of thecoupling agents are low molecular weight polycarbonates, quinones,heterocycles and formals.

The polyphenylene ether generally has a molecular weight (numberaverage, as determined by gel permeation chromatography, whenever usedherein) within the range of about 5,000 to 40,000. The intrinsicviscosity of the polymer is usually in the range of about 0.38 to 0.5deciliters per gram (dl./g.), as measured in solution in chloroform at25° C.

The polyphenylene ethers may be prepared by known methods, and typicallyby the oxidative coupling of at least one correspondingmonohydroxyaromatic (e.g., phenolic) compound. A particularly useful andreadily available monohydroxyaromatic compound is 2,6-xylenol (in whichfor the above formula each Q¹ is methyl and each Q² is hydrogen), thecorresponding polymer of which may be characterized as apoly(2,6-dimethyl-1,4-phenylene ether).

Any of the various catalyst systems known in the art to be useful forthe preparation of polyphenylene ethers can be used in preparing thoseemployed in this invention. For the most part, they contain at least oneheavy metal compound, such as a copper, manganese or cobalt compound,usually in combination with various other materials. Among the preferredcatalyst systems are those containing copper. Such catalysts aredisclosed, for example, in the aforementioned U.S. Pat. Nos. 3,306,874and 3,306,875, and elsewhere. They are usually combinations of cuprousor cupric ions, halide ions (i.e., chloride, bromide or iodide), and atleast one amine. catalyst preferred are catalyst systems containingmanganese. They are generally alkaline systems containing divalentmanganese and such anions as halide, alkoxide or phenoxide. Most often,the manganese is present as a complex with one or more complexing and/orchelating agents such as dialkylamines, alkanolamines, alkylenediamines,o-hydroxyaromatic aldehydes, o-hydroxyazo compounds, alphahydroxyoximes(both monomeric and polymeric), o-hydroxyaryl oximes, andbeta-diketones. Also useful are cobalt-containing catalyst systems.Those skilled in the art will be familiar with patents disclosingmanganese and cobalt-containing catalyst systems for polyphenylene etherpreparation.

Especially useful polyphenylene ethers for the purposes of thisinvention are those which comprise molecules having at least one of theend groups of formulas II and III, below, in which Q¹ and Q² are aspreviously defined, each R¹ is independently hydrogen or alkyl,providing that the total number of carbon atoms in both R¹ radicals is 6or less, and each R² is independently hydrogen or a C₁₋₆ primary alkylradical. Preferably, each R¹ is hydrogen and each R² is alkyl,especially methyl or n-butyl. ##STR3##

Polymers containing the aminoalkyl-substituted end groups of formula IImay be obtained by incorporating an appropriate primary or secondarymonoamine as one of the constituents of the oxidative coupling reactionmixture, especially when a copper- or manganese-containing catalyst isused. Such amines, especially the dialkylamines and preferablydi-n-butylamine and dimethylamine, frequently become chemically bound tothe polyphenylene ether, most often by replacing one of thealpha-hydrogen atoms on one or more Q¹ radicals adjacent to the hydroxygroup on the terminal unit of the polymer chain. During furtherprocessing and/or blending, the aminoalkyl-substituted end groups mayundergo various reactions, probably involving a quinone methide-typeintermediate of formula IV, below (R¹ is defined as above), withbeneficial effects often including an increase in compatibilization withother blend components. ##STR4##

Polymers with bisphenol end groups of formula III are typically obtainedfrom reaction mixtures in which a by-product diphenoquinone of formulaV, below, is present, especially in a copper-halide-secondary ortertiary amine system. In this regard, the disclosures of the U.S. Pat.Nos. 4,234,706, 4,477,649 and 4,482,697 are particularly pertinent. Inmixtures of this type, the diphenoquinone is ultimately incorporatedinto the polymer in substantial amounts, chiefly as an end group.##STR5##

In many polyphenylene ethers obtained under the conditions describedabove, a substantial proportion of the polymer molecules, usually asmuch as about 90% by weight of the polymer, contain end groups havingone or frequently both of formulas II and III. It should be understood,however, that other end groups may be present and that the invention inits broadest sense may not be dependent on the molecular structures ofthe polyphenylene ether end groups.

It will thus be apparent to those skilled in the art that a wide rangeof polymeric materials encompassing the full recognized class ofpolyphenylene ether resins are contemplated as suitable for use in thepractice of the present invention.

In order to cause rapid absorption of the vinyl aromatic monomer, thepolyphenylene ether is used in a particulate form. It is preferably inthe form of powder or spheres, particles or pellets having a diameter ofabout 0.2 to 10 mm.

Examples of vinyl aromatic monomers are used in the process of thisinvention are sytrene, alpha-methylstyrene, ethylstyrene, chlorostyrene,bromostyrene, vinyltoluene, vinylbenzene, and isopropylxylene. Thesemonomers may be used either alone or in admixture. A mixture of at least50% of the vinyl aromatic monomer and a monomer copolymerizable with it,such as acrylonitrile, methyl methacrylate or methyl acrylate can alsobe used.

The polymerization catalysts used in the process of this inventioninclude, for example, organic peroxides such as benzoylperoxide,tertiary butyl perbenzoate, lauroyl peroxide, tertiary butylperoxy-2-ethylhexanate and tertiary butyl peroxide, and azo compoundssuch as azobisisobutyronitrile and azobisdimethylvaleronitrile.

In the process of this invention, a cross-linking agent is not alwaysnecessary but may be added. Examples of such cross-linking agents aredi-t-butyl peroxide, t-butyl cumyl peroxide, dicumyl peroxide, alpha,alpha-bis(t-butyl peroxy)p-di-isopropylbenzene,2,5-dimethyl-2,5-di(t-butyl peroxy) hexyne-3,2,5-dimethyl-2,5-di(benzoylperoxy)hexane, and t-butyl peroxy isopropyl carbonate. The cross-linkingagent is used generally together with a cross-linking promoter. Examplesof cross-linking promoters include functional vinyl compounds such asdivinylbenzene, polyethylene glycol dimethacrylate, triallyl cyanurate,diallyl phthalate, 1,3-butadiene, 1,2-polybutadiene and1,4-polybutadiene, quinone dioxime, and bisamide.

The process of this invention is conventiently carried out as follows.First, polyphenylene ether resin (to be referred to as a nuclear resin)is reduced to powder by, for example, precipitation from a reactionmixture. The powder of the nuclear resin is suspended in an aqueousmedium containing a dispersing agent. The dispersing agent may be, forexample, polyvinyl alcohol, methyl cellulose, calcium phosphate,magnesium pyrophosphate, calcium carbonate, etc. The amount of thedispersing agent employed is 0.01 to 5% by weight based on the amount ofwater. Then a vinyl aromatic polymerization catalyst is added to theresulting suspension containing the nuclear resin particles dispersedtherein. These materials may be added all at one time, or gradually insmall portions. The vinyl aromatic monomer and the polymerizationcatalyst may be added separately. Alternatively, the nuclear resin andthe polymerization catalyst may be first dissolved in, or mixed with,the vinyl aromatic monomer and the solution may be used as a solution ina solvent which does not hamper the polymerization reaction. Examples ofsolvents that can be used for this purpose include toluene, benzene and1,2-dichloropropane.

In one embodiment of the present invention, the aqueous medium is heatedto a temperature at which the vinyl monomer can be polymerized, and thenthe vinyl aromatic monomer and the polymerization catalyst are added.Alternatively, these materials are added to room temperature, and thenthe suspension is heated to the polymerization temperature. When thecross-linking agent is used in the process of this invention, it may beused for dissolving in the vinyl aromatic monomer, or in the solvent forthe polymerization catalyst. When the vinyl aromatic monomer is used ina relatively large amount, it is desirable to add the vinyl aromaticmonomer gradually in small portions to the suspension in order toprevent the formation of a homopolymer of the vinyl aromatic monomer.

The vinyl aromatic monomer added to the suspension penetrates into thedissolved nuclear resin chains and is there polymerized, or polymerizedand crosslinked, in the nuclear resin chains. As the result of thisreaction, interpolymerization of the vinyl aromatic monomer takes place.In this reaction, 1 to 50, preferably 5 to 40 parts by weight of thenuclear resin and 99 to 50, preferably 95 to 60 parts by weight of thevinyl aromatic monomer are used. When the amount of the vinyl aromaticmonomer is smaller than that above-specified, the solution is veryviscous and hard to interpolymerize. On the other hand, when the amountof the vinyl aromatic monomer is larger than the upper limit specified,elasticity, thermal stability and oil resistance of the resulting foamedproduct tend to be deteriorated. Accordingly, the proportions of thepolyphenylene ether resin and the vinyl aromatic monomer employed shouldbe in the range of from 1 to 50 parts by weight, preferably from 5 to 40parts by weight, of the polyphenylene ether resin, and from 99 to 50parts by weight, preferably from 95 to 60 parts by weight, of the vinylaromatic monomer, per 100 parts by weight of resin and monomer.

The resulting thermoplastic resin particles consist of a polyphenyleneether resin interpolymerized with a vinyl aromatic homopolymer. Sincethe above reaction provides thermoplastic resin beads containing theinterpolymer, phase separation as seen in many polymer blends does notoccur, and, thus, the interpolymer exhibits the effect of increasingcompatibility between the polyphenylene ether resin and the vinylaromatic polymer.

In the process of this invention, for example, a blowing agent isimpregnated under pressure in the resulting thermoplastic resin beads inan aqueous suspension. A suspending agent is preferred to be added tothe aqueous suspension in order to prevent bonding or coalescing of thethermoplastic resin beads during impregnation with the blowing agent.Examples suspending agents are organic compounds such as polyvinylalcohol, polyacrylic acid salt, polyvinyl pyrrolidone, carboxymethylcellulose, calcium stearate and ethylene-bis stearamide, and, sparingly,water-soluble fine powders of inorganic compounds such as calciumpyrophosphate, calcium phosphate, calcium carbonate, magnesiumcarbonate, magnesium phosphate, magnesium pyrophosphate and magnesiumoxide. When an inorganic compound is used as the suspending agent in theprocess of this invention, it should be desirably used together with asurface active agent such as sodium dodecylbenzenesulfonate.

Easily volatilizable blowing agents are used in the process of thisinvention. Examples of blowing agents include aliphatic hydrocarbonssuch as propane, n-butane, i-butane, n-pentane isopentane and n-hexane;cycloaliphatic hydrocarbons such as cyclopentane and cyclohexane; andhalogenated hydrocarbons such as methyl chloride, ethyl chloride,dichlorodifluoromethane, chlorodifluoromethane andtrichlorofluoromethane. These blowing agents are used in an amount ofgenerally in the range of from 1 to 40, preferably up to 30 parts byweight based on 100 parts by weight of the thermoplastic interpolymerresin beads and blowing agent. A small amount for example, 1 to 5% byweight, of an organic solvent such as toluene or xylene, may be usedtogether therewith.

The impregnation of the blowing agent is performed, for example, bysuspending the polymerizable ingredients in water containing thesuspending agent in an autoclave, heating the suspension, andintroducing the blowing agent, e.g., under pressure, before or after theinterpolymer beads are formed. This procedure affords expandablethermoplastic resin beads.

The blowing agent impregnated in the expandable thermoplastic resinbeads obtained by the process of this invention does not easily escapefrom the beads, and the resin beads can be stored in a stable conditionover long periods of time. As a result, it is not necessary, to pre-foamthe beads within 24 hours after preparation, or to store them in acontainer under pressure. Therefore, the resulting expandablethermoplastic resin beads can be stored or transported as prepared. Forexample, when the expandable resin particles, in accordance with thisinvention, are prefoamed with steam after one week storage atatmospheric pressure, foamed beads having a sufficient expansion ratiocan be obtained.

A foamed article having a high expansion factor can be prepared byheating the expandable thermoplastic resin beads in accordance withdisclosure of this invention, by a heating medium, such as steam.

The expandable thermoplastic resin beads obtained by the process of thisinvention can be formed into a foamed shaped article of a desiredconfiguration by pre-foaming the beads and fusing them in a mold cavity.The resulting foamed shaped article has superior thermal stability,chemical resistance (e.g., oil resistance) and compressive strength dueto the formed interpolymer. In particular, when the foamed article isused as an underlayer of a roofing material to be subject to hightemperatures, it is not shrunk nor softened by heat, and, therefore, itfinds extensive use as a heat or sound insulating material or acushioning material.

In one way of carrying out the process of this invention, the blowingagent is impregnated after the thermoplastic resin beads have beenprepared. It is not necessary to use a high pressure reactor forpolymerization and, optionally, crosslinking, and polymer beads can bevery easily obtained. According to the process of this invention,therefore, thermoplastic resin beads can be obtained prior to theimpregnation of the blowing agent by polymerizing the vinyl aromaticmonomer with the nuclear resin in the presence of a polymerizationcatalyst and, optionally, a cross-linking agent to induceinterpolymerization or both interpolymerization and cross-linking. Theseresin beads can be formed into expandable thermoplastic resin beads inthe manner described hereinabove. These resin beads can also be used asa resin for extrusion shaping. For example, it is possible to feed thesebeads into an extruder, force a blowing agent into it, and extrude afoamed sheet, board or rod.

Furthermore, according to the process of this invention, it is possibleto add a fire retarding agent, a coloring agent, an antistatic agent,etc.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following Examples illustrate the present invention. Unlessotherwise specified, all parts and percentages are by weight.

EXAMPLE 1

Beads of poly(2,6-dimethyl-1,4-phenylene ether) (PPO®)/styreneinterpolymer were produced via suspension polymerization conducted inthe following manner. One hundred grams of deionized water was added toa three neck glass reaction vessel equipped with a thermometer,condenser, nitrogen purge and mechanical stirrer. Throughout, thereaction the vessel was constantly stirred at 800-1000 rpms and allsolutions were kept under nitrogen. To the reaction vessel 73 grams of asolution of 20 parts or poly(2,6-dimethyl-1,4-phenylene)ether PPOdissolved in 80 parts of styrene monomer was added followed by 54 gramsof 0.3% polyvinyl alcohol aqueous solution. After 30 minutes,azobisisobutylnitrile (AIBN) (1.0 g) dissolved in acetone and 16 gramsof pentane were added. The temperature of the vessel was heated to 75°C. for 15-18 hours following distillation of the acetone.

The reaction mixture was cooled, filtered, and the resultant granuleswere washed several times with deionized water then air dried overnight.The granules had a diameter of predominantly less than 1.5 mm by opticalmicroscopy and had a glass transition temperature after pre-expansion of117° C. by differential scanning calorimetry. Bulk density of thepre-expanded material (40 psi for 30 seconds) was approximately 2.25lbs./ft.³.

EXAMPLE 2

The suspension polymerization was carried out in the same fashion asExample 1, with a few minor changes. The three neck glass reactionvessel equipped with a thermometer and stirrer was purged for 15 minutesbefore the addition of 100 grams of deionized water and 50 grams of 0.3%polyvinyl alcohol aqueous solution. After an hour 80 grams of 20/80PPO®/styrene monomer and 1.0 g AIBN and 16 g of pentane were slowlyadded to the reaction vessel. After an additional hour of purging, thevessel was heated to 75°-80° C. for 17-18 hours.

The reaction mixture was allowed to cool and spherical beads werecollected by filtration and washed with deionized water then air driedovernight. The beads had a diameter of primarily 1.5-2.0 mm via opticalmicroscopy and a glass transition temperature of 104° C. by means ofdifferential scanning calorimetry. Bulk density of the pre-expandedmaterial (40 psi for 30 seconds) was roughly 10.5 lbs./ft.³.

EXAMPLE 3

The pre-expanded imbibed particles of Example 1 can be formed into afoam structure as follows: The pre-expanded particles are loaded into amold of a suitable size, typically, 12×12×0.5 inches. Steam isintroduced into the mold at a pressure of 60 psi. The confined expandedpellets assume the shape of the mold. Their final density is less than 5lbs./ft.³. The foamed shape has excellent physical, thermally resistant,solvent-resistant and oxidation-resistant properties.

EXAMPLE 4

The particles of Example 1 are extruded at 450° F. through a circularslit die to yield a sheet of about 90 mils thickness, density about 2.5lbs./ft.³, closed cell structure. The foam sheet can be preheated to atemperature of about 475°-525° F. and incrementally advanced to male andfemale dies which will conform the resin sheet into a plurality ofsemi-circular sheaths of two of which may accommodate the insulation ofa conduit having an outside diameter of about 1 inch. After thestructures are thermoformed in the foam sheet, the molds are cooled, thethermoformed sheet removed and the impressed structures are separatedfrom the selvage of the sheet.

The above-mentioned patents are incorporated by reference.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof. For example, instead ofpoly(2,6-dimethyl-1,4-phenylene ether) there can be usedpoly(2,6-dimethyl-co-2,3,6-trimethyl-1,4-phenylene ether). Apolyphenylene ether resin capped with an ester or an ether group can beused, see, e.g., Holoch et al., U.S. Pat. No. 3,375,228, and Hay andWhite, U.S. Pat. No. 4,048,143, both incorporated herein by reference.Instead of styrene monomer, there can be used alpha-methyl styrene,bromostyrene, chlorostyrene, vinyltoluene, mixtures of any of theforegoing and the like. Instead of interpolymerizing with the catalyst,preformed interpolymer particles can be suspended in water and imbibedwith the blowing agent. The interpolymer beads can be imbibed by dippingthe beads into the liquid blowing agent. Instead of pentane, the blowingagent can comprise n-butane, methyl chloride, dichlorodifluoromethane,chlorodifluoromethane, trichlorofluoromethane, mixtures thereof, and thelike. All such obvious variations are within the full intended scope ofthe appended claims.

We claim:
 1. A thermoformed structure of an interpolymer of from 1 to 50parts of a polyphenylene ether resin and from 99 to 50 parts of apolymerized vinyl aromatic monomer per 100 parts of resin and monomerfoam prepared by the method which comprises:(a) subjecting a sheet ofsaid foam having a density of less than 20 lbs./ft.³ to a firsttemperature sufficient to permit a shaped formation to be effectedtherein; (b) effecting a shape in said sheet while at said temperature;and (c) reducing the temperature of said sheet to a second temperaturepermitting permanent retention of said shape in said sheet at or belowsaid second temperature.
 2. A thermoformed structure as defined in claim1 wherein said interpolymer comprisespoly(2,6-dimethyl-1,4-phenylene)ether resin in polymerized styrenemonomer.
 3. The thermoformed structure of claim 1 wherein theinterpolymer comprisespoly(2,6-dimethyl-co-2,3,6-trimethyl-1,4-phenylene ether) resin inpolymerized styrene monomer.
 4. The thermoformed structure of claim 1wherein the interpolymer comprises a copolymer resin containing2,6-dimethyl-1,4-phenylene ether units and 2,3,6-trimethyl-1,4-phenyleneether units, in polymerized styrene monomer.
 5. The thermoformedstructure of claim 1 wherein the polyphenylene ether resin in theinterpolymer is one which has been capped with an ester group or anether group.
 6. The thermoformed structure of claim 1 wherein the vinylaromatic monomer is selected from the group consisting of styrene,alpha-methylstyrene, ethylstyrene, chlorostyrene, bromostyrene,vinyltoluene, vinylbenzene, isopropylxylene, and mixtures of any ofthese monomers.
 7. The thermoformed structure of claim 6 wherein themonomer mixture comprises 50% of the vinyl aromatic monomer, and atleast one second monomer selected from the group consisting ofacrylonitrile, methyl methacrylate, and methyl acrylate.